1. Field of the Invention
Aspects of the present invention relate to a polymer electrolytic membrane, and a fuel cell employing the same, and more particularly, to a polymer electrolytic membrane having excellent ion conductivity, chemical and mechanical stabilities and a fuel cell employing the same.
2. Description of the Related Art
Fuel cells may be classified according to their electrolyte type. Types of fuel cells include polymer electrolyte membrane fuel cells (PEMFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC) and others. The working temperatures of fuel cells and their constituent materials vary depending on the electrolyte type.
According to the method of supplying fuel to the anode, fuel cells can be classified into external reforming type fuel cells, in which fuel is converted into hydrogen enrichment gas by a fuel reformer and supplied to the anode, and internal reforming type fuel cells, in which fuel in a liquid or gaseous state is directly supplied to the anode.
A representative example of a direct fuel supply type fuel cell is a direct methanol fuel cell (DMFC). In the DMFC, an aqueous methanol solution is used as fuel, and a proton conductive polymer electrolyte membrane is used as an electrolyte. Accordingly, DMFC is a kind of PEMFC.
PEMFCs are small and lightweight but can achieve a high output density. Furthermore, a power generation system can be easily constituted using a PEMFC.
The basic PEMFC may include an anode (fuel electrode), a cathode (oxidizing agent electrode), and a polymer electrolyte membrane interposed between the anode and the cathode. The anode may include a catalyst layer to promote the oxidation of fuel and the cathode may include a catalyst layer to promote the reduction of an oxidizing agent.
The fuel supplied to the anode may be hydrogen, a hydrogen-containing gas, a mixture of methanol vapor and water vapor, an aqueous methanol solution, or the like. The oxidizing agent supplied to the cathode may be oxygen, an oxygen-containing gas, air, or the like.
Fuel is oxidized to produce protons and electrons at the anode of the PEMFC. The protons migrate to the cathode through an electrolyte membrane, and the electrons migrate to an external circuit (load) through a conductive wire (or current collector). At the cathode of the PEMFC, the migrated protons react with the electrons and oxygen to produce water. The migration of electrons from the anode to the cathode via the external circuit generates electric power.
In a PEMFC, the polymer electrolyte membrane acts as an ionic conductor for the migration of protons from the anode to the cathode and also acts as a separator to prevent contact between the anode and the cathode. The polymer electrolyte membrane therefore requires sufficient ionic conductivity, electrochemical safety, high mechanical strength, thermal stability at its operating temperature, and should be easily formed into thin layers.
Materials for the polymer electrolyte membrane typically include a sulfonated perfluorinated polymer with fluorinated alkylene in the backbone and fluorinated vinylether side chains with sulfonic acid at its terminal. An example of a sulfonated perfluorinated polymer is NAFION, manufactured by Dupont. A polymer electrolyte membrane impregnated with an appropriate amount of water provides excellent ionic conductivity.
However, such a polymer electrolyte membrane may have insufficient methanol permeability and a high cost of manufacturing and may experience a lowered ionic conductivity at operating temperatures of 100° C. or higher due to the loss of moisture by evaporation. It is therefore difficult to operate a PEMFC using such polymer electrolyte membrane under atmospheric pressure at about 100° C. or higher. PEMFCs have been operated at 100° C. or lower, such as, for example, at about 80° C.
When an electrolyte membrane has high ion conductivity, the water permeability of the electrolyte membrane is high, and thus the methanol permeability also tends to be high. Therefore, an electrolyte membrane having high ion conductivity cannot easily have low methanol permeability. The amount of water and methanol passing through an electrolyte membrane in an aqueous methanol solution having a predetermined concentration can be measured and compared with the amount passing through a standard electrolyte membrane, such as, for example NAFION 115. When the amount of water passing through the electrolyte membrane is the same as or more than that passing through Nafion 115 and the amount of methanol is the same as or less than that passing through Nafion 115, the electrolyte membrane is considered to be suitable as an electrolyte membrane for a DMFC.
To meet these requirements, much research on polymer electrolyte membranes capable of replacing the NAFION electrolyte membrane has been carried out. Block copolymers including hydrocarbon based repeating units such as styrene repeating units, ethylene-r-butylene repeating units, and isobutylene repeating units have been known as materials for polymer electrolyte membranes.
However, such block copolymers fail to stabilize membrane and electrolyte assembly (MEA) values due to methanol crossover and swelling.